The invention relates to a superconducting wire, comprising NbTi superconducting material and Cu.
Niobium-titanium (NbTi) is an important superconducting material, which, due to its good plastic deformability (both of a final material and of its precursors), can be deployed in a variety of ways, for example, in superconducting magnet coils or in superconducting cables. The NbTi superconducting material is typically integrated in a superconducting wire, wherein a multiplicity of NbTi filaments usually extends in the longitudinal direction of the superconducting wire.
As part of a drawing process, the wire manufacturing procedure usually includes plastic deformation in a hexagonal outside cross-section in order to simplify the bundling processes of individual NbTi filaments (or also other structures and intermediate structures). The resulting hexagonal elements can then be very compactly disposed, for example, in a copper enclosing tube, cf. U.S. Pat. No. 5,088,183 A referred to in the introduction
The superconducting properties of NbTi are only achieved at especially low temperatures, below approx. 9 K, so that the superconductor must be cooled, for example, with liquid helium. Moreover, the superconducting condition cannot be maintained if the magnetic field strength or the electric current density is too high.
NbTi superconducting wires are typically stabilized with copper (Cu), usually with a high degree of purity. The copper runs parallel to the superconducting filaments in the superconducting wire; for example, the copper can constitute a matrix for superconducting NbTi filaments. Copper is a good heat conductor, through which effective cooling of the superconducting NbTi filaments can be achieved. Furthermore, copper exhibits high electrical conductivity. If superconductivity in the superconducting wire is lost locally, the copper provides a parallel electrical current path, limiting a rise in electrical resistance and associated heat generation. With effective cooling, superconductivity can be restored to the local, normally conducting region in the superconductor (stabilizing function). Even if superconductivity cannot be restored, the copper protects the superconductor from damage (“melting”) by limiting the increase in resistance and thus limiting heat generation, while also efficiently removing the resulting heat (quench protection). Moreover, the copper improves the mechanical strength of the superconducting wire.
Not only is copper an expensive material, it is also relatively heavy with a density of approx. 8.9 g/cm3, which is undesirable for many applications. Alternative materials that can at least partially replace copper are therefore sought for stabilizing NbTi superconductors. Pure aluminum (Al) would be a suitable material in terms of heat conductivity and electrical resistance, and is less expensive than copper and, with a density of 2.7 g/cm3, considerably lighter.
WO 2008/121764 A1 proposes a NbTi superconductor structure with a Cu sleeve, an Al core and multifilament NbTi rods disposed in a ring between them.
While Cu and NbTi are both relatively hard and behave in a similar way when plastically deformed, Al is much softer than NbTi and Cu. When wire containing Al structures, in particular, an Al core, was drawn, cracks extending transversely with respect to the longitudinal direction of the wire were observed in an NbTi superconducting wire. A wire containing cracks is of no use for superconductor applications.
U.S. Pat. No. 5,189,386 discloses a superconductor structure in which an Al core is divided into six sectors by layers of a Cu—Ni alloy. This structure is relatively complex.
U.S. Pat. No. 4,652,697, which is the closest prior art, describes a superconductor structure in which three Al wires and four multifilament wires are bundled, for example, with NbTi filaments and a copper matrix. One of the multifilament wires is disposed in the center of the superconductor structure. The structure is held together by PbSn solder. It is difficult to process solder in wire production and, as an additional material, it can additionally aggravate the problem of crack formation.
JP 09 282953 A describes an Nb3Al superconductor wire in which Nb3Al filaments are embedded in a copper matrix. Embedded additional structures made of Nb or Ta can be used for strengthening. In one embodiment, additional structures with an Al rod in a Cu tube are proposed.
The object of the invention is to propose a stabilized NbTi superconducting wire, which has a low weight, which can be manufactured at low cost, wherein the superconducting wire has a reduced risk of crack formation (especially during wire drawing).